© 2019 CERN. for the LHCb Collaboration. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. The decay of the narrow resonance Bs2∗0→B-K+ can be used to determine the B- momentum in partially reconstructed decays without any assumptions on the decay products of the B- meson. This technique is employed for the first time to distinguish contributions from D0, D∗0, and higher-mass charmed states (D∗∗0) in semileptonic B- decays by using the missing-mass distribution. The measurement is performed using a data sample corresponding to an integrated luminosity of 3.0 fb-1 collected with the LHCb detector in pp collisions at center-of-mass energies of 7 and 8 TeV. The resulting branching fractions relative to the inclusive B-→D0Xμ-νμ are fD0=B(B-→D0μ-νμ)/B(B-→D0Xμ-νμ)=0.25±0.06,fD∗∗0=B(B-→(D∗∗0→D0X)μ-νμ)/B(B-→D0Xμ-νμ)=0.21±0.07, with fD∗0=1-fD0-fD∗∗0 making up the remainder.